Concentrations of sodium, potassium, calcium, chloride and materials in solutions and the determination of pH can be made electrically with electrodes that are selective to ions of those materials. The standard reference text in this field is Reference Electrodes Theory and Practice by Ives and Janz. That work and the patent literature make it clear that measurement is difficult. The potentials are minute, they vary with temperature, the measuring circuit necessarily includes junctions between dissimilar materials and metal to liquid junctions across which interfering potentials appear at magnitudes comparable to the potentials to be measured.
Practical measurement has been characterized by short term drift in potentials and need for frequent calibration, often in reference solutions which differ in large degree from the composition of the test solution, and that results in error and uncertainty about accuracy of measurements. Attempts to overcome these difficulties have concentrated on design of voltmeters, on finding new or improved ion selective "membrane" materials, and on design of the electrode structure primarily in an effort to avoid "poisoning" and fouling of electrodes and to minimize the effect of interfering ions by the sample being measured. The price for improvement is usually higher cost, but the advantage of such improvements is often lost in the need to extend the use of the more expensive electrode over longer periods or a greater number of tests.
The prior art is illustrated in FIGS. 2 and 3 of the drawing. The two electrodes, one the reference electrode and the other the ion selective electrode, are placed in a first body of electrolyte called a standard solution, and the voltmeter is calibrated. Then the electrolyte is removed and is replaced with a second body of standard solution and the slope of the voltmeter reading change is adjusted. Thereafter, the second body of electrolyte is removed and replaced with the sample to be tested. In that process each measurement is a separate event in which the measuring circuit is reconstructed both on the reference side and on the ion selective side. Temperatures of the solutions and the sample may vary, and a relatively large amount of sample is required. The measurement steps are usually separated by rinsing steps in which there is opportunity to alter junction potential equilibria at the electrodes.
Special electrodes and techniques have been developed in an effort to speed the measurement process and improve accuracy. FIG. 3 shows one of the more advanced of those developments. The ion selective electrode is inverted and inserted in the lower end of a tube. The ion selective membrance forms the bottom wall of a sample cup into which the reference electrode is lowered as the cup is filled successively with mid-range reference solution, rinse material, end-range reference solution, rinse material, and sample solution. This structural arrangement has some advantages, but the major causes of error and uncertainty remain. The circuit is interrupted both on the reference electrode side and on the ion selective side, and there is opportunity for error caused by liquid-junction-potential variations at both the measuring and reference electrode.